The present invention relates generally to spinal implants for loadbearing applications. In certain aspects, the present invention relates to synthetic spinal implants configured for placement between two adjacent vertebrae to facilitate fusion.
As further background, intervertebral discs, located between the endplates of adjacent vertebrae, stabilize the spine, distribute forces between vertebrae, and cushion vertebral bodies. A normal intervertebral disc includes a semi-gelatinous component, the nucleus pulposus, which is surrounded and confined by an outer, fibrous ring called the annulus fibrosus. In a healthy, undamaged spine, the annulus fibrosus prevents the nucleus pulposus from protruding outside the disc space.
Spinal discs may be displaced or damaged due to trauma, disease or aging. Disruption of the annulus fibrosus allows the nucleus pulposus to protrude into the vertebral canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on a spinal nerve, which may result in nerve damage, pain, numbness, muscle weakness and paralysis. Intervertebral discs may also deteriorate due to the normal aging process or disease. As a disc dehydrates and hardens, the disc space height will be reduced leading to instability of the spine, decreased mobility and pain.
In certain instances, the only relief from the symptoms of these conditions is a discectomy, or surgical removal of a portion or all of an intervertebral disc, followed by fusion (arthrodesis) of the adjacent vertebrae. The removal of the damaged or unhealthy disc will allow the disc space to collapse. Collapse of the disc space can cause instability of the spine, abnormal joint mechanics, premature development of arthritis or nerve damage, in addition to severe pain. Pain relief via discectomy and arthrodesis requires preservation of the disc space and eventual fusion of the affected motion segments.
Historically, bone grafts have been used to fill the intervertebral space to promote fusion of the adjacent vertebrae across the disc space. In early techniques, bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebra, and the spinal column was stabilized by way of a plate or rod spanning the affected vertebrae. Once fusion occurred, the hardware used to maintain the stability of the segment became superfluous and was a permanent foreign body. Moreover, the surgical procedures necessary to implant a rod or plate to stabilize the level during fusion were frequently lengthy and involved.
A variety of attempts have been made to develop implants for use in maintaining the disc space until complete arthrodesis is achieved. The implant must provide temporary support and allow bone ingrowth. Success of the discectomy and fusion procedure requires the development of a contiguous growth of bone to create a solid mass because the implant may not withstand the compressive loads on the spine for the life of the patient.
As examples, several metal spacers have been developed to fill the void between adjacent vertebral bodies and to promote fusion. These include hollow spinal cages that can be filled with osteogenic material, such as autograft, allograft, or osteogenic protein formulations, prior to insertion into the intervertebral space. Apertures defined in the cage communicate with the hollow interior to provide a path for tissue growth between the vertebral endplates.
Interbody spinal implants fabricated from bone have also been employed. These include for instance threaded bone dowel products and impacted spacers. Bone provides many advantages for use in fusions. It can be incorporated after fusion occurs and therefore will not be a permanent implant. Bone also allows excellent postoperative imaging because it does not cause scattering like metallic implants. Stress shielding is avoided because bone grafts have a similar modulus of elasticity as the surrounding bone.
Although an all-bone spacer provides these and other benefits, the use of bone presents several challenges. Any spacer which will be placed within the intervertebral disc space must withstand the cyclic loads of the spine. Cortical bone products may have sufficient compressive strength for such use; however, cortical bone will not promote rapid fusion. Cancellous bone is more conducive to fusion but is not biomechanically sound as an intervertebral spacer. As well, suitable allograft bone can be relatively scarce at times, potentially interrupting product supply.
In view of the background in this area, needs remain for improved and alternative intervertebral spacer implants that are fabricated from readily available materials and which have the mechanical and biological attributes necessary for loadbearing spinal applications. The present invention, in certain embodiments, is addressed to these needs.